WO1998036234A1 - Camouflage structure - Google Patents

Abstract

For a camouflage structure not to loose its effectiveness against infrared surveillance even in variable temperature conditions (day/night, exposure to sun/cloudy weather), it has emissivity curves of different tendencies in the atmospheric windows II (3-5 mu m) and III (8-14 mu m), i.e. the emissivity in the infrared range is not just constant at a particular level but has a rising or falling tendency in at least one selected spectral range. It is particular advantageous for the emissivity curve to have a falling tendency in the atmospheric window II.

Description

 Camouflage structure

Technical field

The invention relates to a camouflage structure with a layer reflecting in the IR range, and to a camouflage network with such a structure. State of the art

The most comprehensive camouflage of objects, systems and people is a central aspect of every military defense system. The aim is to prevent or at least complicate the reconnaissance in the visible, (near and far) IR range (IR = infrared) and preferably also in the radar range. In principle, camouflage layers that perform this task more or less well have been known for a long time.

In order to be able to achieve a good camouflage coating, the camouflage effect must of course extend to the entire wavelength range that can be detected by sensors. In the infrared, the spectral range covering atmospheric windows II (3 - 5 μm) and III (8 - 14 μm) must be taken into account (see e.g. Electro-Optics Handbook, Technical Series EOH-11, RCA Corporation, 1974, p. 91, paragraph 2).

A camouflage coating with an effective broadband effect from the visible to at least the IR spectral range is already known from GB-565.238. The camouflage effect is achieved in that an upper coating, which is responsible for the camouflage in the visible range, is made transparent for infrared radiation, and that an underlying primer reflects the infrared radiation in the desired manner.

The known coating thus consists of a primer and a camouflage paint (pigment layer) applied to it, which reflects in the visible area like the natural background (for example chlorophyll). The primer is reflective in the area of terrestrial thermal radiation and the cover layer is transparent for this spectral area. The pigment layer must therefore use a binder that has good transparency in the spectral ranges of atmospheric windows II and III. DE-PS 977 526 discloses camouflage which is effective in visible light, in the infrared region and in the case of radar sighting. For camouflage in the radar area, a camouflage net is provided with an electrically conductive underlayer (primer). It can either be a metal paint (metallic color) or a glued-on metal foil. In any case, the primer is designed so that it is well reflective in the relevant wavelength range. As a result, the homogeneous metallic primer (due to the low surface resistance of at most a few ohms) reflects well in the radar range. Scattering and absorbing layers are applied to the primer. The top layer is preferably a camouflage paint which is effective in the visible range and is applied in a manner known per se.

Another camouflage coating is known from DE 725 253. For an optimal camouflage, which extends over the visible as well as the long-wave area, an underlay of the visible camouflage coating with a layer reflecting in the long-wave area is suggested (see eg page 2, lines 19 - 32), which consists of a Metal foil (see page 2, Ex. 4) or a metallic color (see page 2, lines 33 - 43). An aluminum foil (because it forms a homogeneous metallic coating) has a very good conductivity, i.e. a strong reflective effect for electromagnetic radiation in the radar range. The known coating is thus designed so that it also automatically reflects in the radar range.

To improve camouflage in the radar range, foils with slits can be used (see e.g. US 3.069.796 or DE 1.088.843).

The later attempts to implement improved camouflage (cf. for example EP 0 058 210, Pusch) did not go much beyond the technical principles known from the abovementioned prior art, so that there is still a need for reconnaissance-resistant camouflage agents. Presentation of the invention

The object of the invention is to provide a camouflage structure which does not lose its effectiveness compared to clearing in the IR range, even under changing temperature conditions (day / night, sunshine / cloud cover)

The solution to the problem is defined by the features of claim 1. According to the invention, the camouflage structure in atmospheric windows II and III tends to have a different course of emissivity in other words. In other words, emissivity in the IR range is not simply constant at a certain level Level, but has an increasing or decreasing tendency in at least one spectral range, with atmospheric windows II and III being of particular importance

With the camouflage structure according to the invention, the thermal behavior (i.e. the black body spectrum) of the ground is imitated, both when exposed to sunlight and when there is cloudiness.There is an essential difference to camouflage structures, which assume the temperature (or the IR spectrum) of the air layer close to the ground The clear sky means that the temperature profile of the ground is significantly different from that of the air.There is also the fact that the temperature distribution of the air is much narrower than that of the ground Adaptation to the floor temperature

An important finding for the camouflage according to the invention is that the zenith temperature is a significant factor for the floor temperature or for its imitation. The goodness of the camouflage depends on how the zenith temperature is reflected However, these are not constant in the IR range, but are dependent on the wavelength. The basic finding is therefore that a camouflage structure has to be spectrally adapted, whereby the circumstances have to be taken into account by an appropriately adapted tendency to emissivity if the camouflage effect is to go beyond the known.

Experiments have shown that it is particularly advantageous if the course of the emissivity in the atmospheric window II tends to decrease. The emissivity is therefore chosen so that - within the window II mentioned - it is higher at small wavelengths than at large ones. The advantageous effect of this measure is in particular also related to the fact that the black body spectrum of the sun drops by about a decade in the range of 3 to 5 μm. However, it is not necessary for the emissivity of the camouflage structure to decrease to the same extent. It is enough if it follows this trend.

Good results can be achieved if the emissivity in the upper wavelength range of the atmospheric window II is at least 25%, in particular approximately 50% lower than in its lower wavelength range. In this way, an undesirable gloss effect (not corresponding to the natural or real background) of the camouflage coating can be minimized.

In the atmospheric window III (in particular in the range of 8-14 μm), the spectral emissivity should be slightly reduced. The trend can be constant. In this sense, the value of the relative emissivity can range between 0.7 - 0.9 (e.g. around 0.8).

At night, the camouflage effect may are adversely affected by the fact that the tendency towards a low zenith temperature is reflected too strongly, which becomes apparent as a "black hole" in the reconnaissance.

In the wavelength range between windows II and III (where the atmosphere is opaque to IR radiation), the emissivity should be as high as possible. It is advantageously higher than in the atmospheric window IM. The camouflage structure according to the invention has at least two layers. The lower one is reflective in the IR range. The upper one mainly consists of a material that is transparent in atmospheric window II, but not in window III.

The top layer is e.g. a pigment coating, which is responsible for the camouflage in the visible area. The above-mentioned material of the cover layer, which is only transparent in spectral regions, is then essentially formed by the binder (including the color pigments) (carrier or matrix made of plastic).

The lower layer (primer) mentioned is metallic. Aluminum is a preferred example. The primer can be designed as a metal foil or as a vapor-deposited or sprayed-on layer on a carrier material.

According to a particularly preferred embodiment, the interface of the primer facing the upper layer is structured three-dimensionally, so that the emissivity of the camouflage structure in the atmospheric window II decreases with increasing wavelength. The three-dimensional structure mentioned can be e.g. in that a carrier formed from a fiber material (fabric) is coated with a metallic coating. However, it is also possible to provide a metal foil (or a foil coated with metal) with a fine embossing of the surface. Another possibility is e.g. in using a brushed aluminum sheet as the underlayer.

It can also be advantageous to incorporate scattering bodies in the camouflage structure, which produce diffuse scattering of the incident radiation in the range of 3-5 μm. In this area, depending on the type of incident radiation, smooth metallic surfaces can lead to strong unnatural reflections, so that the camouflage can be cleared up. Known matting agents with a suitable grain size can serve as scattering bodies.

In practice, multispectral camouflage is very often required. This means that it is not sufficient to ensure camouflage in the IR range, but at the same time a dart camouflage can be created. A good camouflage in the radar area can be achieved that on the one hand the resistance of the metallic coating is selected appropriately and on the other hand there is a three-dimensional shape of the camouflaging surface.

The resistance in the radar range must be dimensioned so that radar waves are absorbed to a certain extent. In practice it turns out that the (wavelength-dependent) resistance is preferably in the range of 30-300 ohms. The resistance can be adjusted by the choice of the layer thickness, the material of the layer, the local opening (holes). Instead of attenuating the electrical field, this can also be done by the magnetic field of the radar wave (e.g. by applying a magnetic layer).

In order to create a three-dimensional shape, a sheet cut (as is known, for example, from US 3,069,796 or DE 1,088,843) can be applied to a fabric or laminate. This measure also has an advantageous effect in the IR range, since it also helps to ensure that the zenith temperature is reflected in a wide variety of observation directions.

From the following detailed description and the entirety of the claims, further advantageous embodiments and combinations of features of the invention result.

Brief description of the drawings

The drawings used to explain the exemplary embodiment show:

Fig. 1 is a schematic representation of a camouflage structure with a fabric as

Carrier; 2 shows a schematic illustration of a camouflage structure in the form of a laminate;

3 shows a schematic representation of a curve according to the invention of the spectral emissivity of the camouflage structure.

In principle, the same parts are provided with the same reference symbols in the figures.

Ways of Carrying Out the Invention

Fig. 1 shows the structure of the camouflage structure according to the invention in cross section. A fiber fabric 1 is used as the carrier. This is not only very robust and tear-resistant, but also has a three-dimensionally structured surface (in the micrometer range) 1.1. In principle, the surface 1.1 is formed by a large number of fine, more or less cylindrical fibers (made of polyester or the like), which lie close together and one above the other. This creates a three-dimensionality which can have a scattering effect in the manner described below for infrared radiation 5 μ in the range of 3 m.

The surface 1.1 is covered with a metal coating 2. This can be sprayed on, vapor deposited or possibly even spread on. According to a particularly preferred embodiment, it is used not only for reflection (or scattering) of the infrared radiation, but also for camouflage in the radar range. The necessary adjustment of the conductivity takes place on the one hand through the suitable choice of the material, on the other hand (and above all) by determining the layer thickness. The surface resistance in the frequency range of radar waves is preferably in the range from a few to a few hundred ohms.

Because the (usually very thin) metal coating 2 is applied to a carrier with a three-dimensionally structured surface 1.1, it has a corresponding structuring in the micrometer range itself on its upper side 2.1. At the top is a cover layer 3. Since this is intended to camouflage in the visible wavelength range (in a manner known per se), it is designed as a pigment layer. Depending on the purpose of the camouflage, the color of the pigments will be more in the gray tone or more in the green tone area.

In the sense of a preferred embodiment of the invention, the binder of the pigment layer (which is decisive for the behavior of the top layer 3 in the infrared range) is transparent for wavelengths of 3-5 μm (atmospheric window II), but not so for wavelengths of 8-14 μm (atmospheric window) III).

The transparency of the top layer 3 can be adjusted by the choice of the layer thickness. If the cover layer 3 is sufficiently thin, a certain transparency (and consequently an emissivity at the desired height) can ultimately be achieved in the atmospheric window III.

The camouflage structure according to the invention can also be formed by a laminate. Such is shown by way of example in FIG. 2. The lower layer, which can be applied to a support, not shown, or possibly is a metal foil 4 itself. It is covered with a cover layer 5, which can be of the same design as that described with reference to FIG. 1.

In order to diffusely scatter the incident infrared radiation to a desired extent, scatter bodies 6 are embedded in the cover layer 5 (or at the interface between the metal foil 4 and the cover layer 5). They are particles whose size is at least in the range of the wavelength of interest (3 - 5 μm), so that they can develop a scattering effect. It can be advantageous if the statistical distribution of the particle size is not too narrow (use of polydisperse matting agents).

The layer structure according to the invention is particularly suitable for camouflage nets. These are fabric or foil-like tarpaulins that can be thrown over the objects to be camouflaged. To have a good effect against radar reconnaissance To achieve this, these camouflage nets are preferably provided with a suitable leaf cut. When spread out, the cut leaves stand up and develop a diffuse scattering effect in the radar range.

3 shows a representation of the size S = 1-p (p = reflectivity), which for gray bodies corresponds approximately to the relative emissivity (E _r ) for a camouflage structure according to the invention as a function of the wavelength (λ). Of interest here is only the wavelength range of 3 - 14 μm, which includes the atmospheric windows II and III.

At the lower end of window II (i.e. at 3 μm) the emissivity is slightly less than 1.0 (e.g. between 0.65 and 0.9).

The emissivity decreases with increasing wavelength. In the present example, it falls to almost half of the original value, i.e. to 0.3 - 0.45. The slope of the waste is e.g. at an octave per micron, especially at about a decade per micron. 3 shows a small plateau in the range between 4 μm and 5 μm.

A sharp increase to a maximum level begins at 5 μm. This is preferably at least as high as the emissivity in the atmospheric window III. In the present case, the maximum is in the range of 0.85 - 1.0. The trend of emissivity - after rising to the maximum - tends to be consistently high.

The emissivity should be reduced in the atmospheric window III. In the present example it is between 0.75 - 0.85. The trend is also constant in this wavelength range (i.e. neither rising nor falling).

3 shows only one of many possibilities. In the area between windows II and III in particular, emissivity does not necessarily have to increase to a maximum level. For example, it can also rise slowly and more or less continuously to the level desired in window III. Because the atmosphere in the Range between 5 μm and 8 μm is not permeable, the course of the emissivity in this wavelength range is not very critical for the quality of the camouflage effect.

3 shows a constant course in the atmospheric window III, but a tendency falling or increasing with increasing wavelength is not excluded. Of course, the course in window II can also have a different tendency.

It goes without saying that a specific measurement curve of a camouflage structure according to the invention will fluctuate within certain limits. Smaller modulations cannot be avoided. This is not so important in the invention. The spacious course is important, i.e. the trend of the curve.

Surface areas with different camouflage structures can be combined on a camouflage net (in the manner of a patchwork arrangement). It should be noted here that the emissivity according to the invention should not be met at a single point in the network, but only as a whole (i.e. taking into account a larger area).

Although camouflage nets are the preferred application, it is not excluded that the camouflage structure according to the invention is attached to the surface of a housing of a technical device or a building.

In summary, it can be stated that the invention has created a camouflage structure which, owing to the wavelength-dependent emissivity, is able to develop a camouflage effect which is optimally adapted to the specific circumstances.

Claims

claims 1. camouflage structure with a layer reflecting in the IR range (2; 4), characterized by an emissivity which tends to have a different course in atmospheric windows II (3 - 5 μm) and III (8 - 14 μm) . 2. camouflage structure according to claim 1, characterized in that in the atmospheric window II (3 - 5 microns) the emissivity has a tendency falling with increasing wavelength. 3. camouflage structure according to claim 2, characterized in that the emissivity in the atmospheric window II (3 - 5 microns) drops by at least 25%, in particular by 50% or more. 4. camouflage structure according to one of claims 1 to 3, characterized in that the emissivity in the atmospheric window III (8 - 14 microns) tends to be constant and is in a range of 0.7 - 0.9. 5. camouflage structure according to one of claims 1 to 4, characterized in that the emissivity in the wavelength range between the atmospheric windows II and III is at least as high as in atmospheric window III (8 - 14 μm). 6. camouflage structure according to one of claims 1 to 5, characterized in that above the first layer (2; 4) reflecting in the IR region, an upper, second layer (3; 5) is provided, which consists essentially of a material , which is transparent in the atmospheric window II (3 - 5 μm), in the atmospheric Window III (8 - 14 μm), however, is not. 7. Camouflage structure according to one of claims 1 to 6, characterized in that the first layer (2; 4) reflecting in the IR region consists essentially of metal, in particular aluminum. 8. camouflage structure according to one of claims 2 to 7, characterized in that an upper interface of the first layer (2; 4) is structured three-dimensionally, so that the emissivity in the atmospheric window II (3 - 5 microns) decreases with increasing wavelength. 9. camouflage structure according to one of claims 6 to 8, characterized in that in the upper, or between the upper and the lower layer, scattering bodies (6) are embedded in order to diffuse scattering of incident infrared radiation, in particular in the range of 3 - 5 μm to effect. 10. camouflage structure according to one of claims 1 to 9, characterized in that it has as a cover layer (3; 5) a pigment layer for camouflage in the visible range. 11. camouflage net with a camouflage structure according to one of claims 1 to 10. 12. camouflage net according to claim 11, characterized in that it is designed as a laminate or coated fabric. 13. camouflage net according to one of claims 11 or 12, characterized in that it has a leaf cut for camouflage in the radar area.